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Old 12-04-2015, 03:15 AM
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Join Date: Dec 2009
Posts: 42
722.315 Modulator Pressure & Fluid Change

When I purchased my 1984 300TD, in 2013, it had a poorly shifting transmission. When I looked it over I found a tag identifying it as an MB rebuild. MB Classic center converted the PN on the tag to confirm this was indeed a 722.315. The A/T did look nice and clean and without leaks.

I drained the A/T fluid, including the torque converter, changed the fluid (Febi) and filter, and rigged up a pressure tester that could be viewed in the cabin, at driving speeds. It soon became clear why the car was sold. The modulator pressure was around 30PSI. The specs call for 2.9 Bar, or 42 PSI. I adjusted the pressure to spec at idle and tested it on the road. As far as I could tell the pressure was the same at idle and 30MPH. The car began to shift like new.

Fast forward 30K miles and I just drained and changed the A/T fluid again. I had a bit more time to test and adjust the modulator pressure. It seemed to have gone down by 2 PSI and was around 39-40 at idle. At speed this would go up to 40-41. I made the necessary correction and the A/T feels very nice now. After the fluid change and the minor modulator adjustment, the shifting feel has improved by a few degrees.

Maybe that A/T improvement, by itself, could be insignificant, but in my observation, small improvements on these old cars have a way of reaching that critical mass where the car begins to be a pleasure to drive.

The main point I want to make here is that the pressure at idle will not necessarily be exactly as it is at 30MP and if you have a tester, it is time well spent, IMO, to do it by the book all the way.

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